(i) Field of the Invention
This invention relates to a novel cold-rolling lube oil (which may hereinafter be called "rolling mill oil" for the sake of brevity) for metallic materials, and more specifically to a cold-rolling lube oil for metallic materials, which exhibits good lubricity and surface cleaness and excellent heat-resistant and oxidation-resistant stability upon rolling metallic materials.
(ii) Description of the Prior Art
In recent years, there is a tendency to use a mill-cleaning rolling mill oil in order to omit the cleaning step in cold rolling. The following two properties may be mentioned as critical for such a mill-cleaning rolling mill oil:
(i) The surface of each plate can be kept free from stains by the carbon component of the rolling mill oil upon its annealing, thereby providing beautiful surface quality (this property will hereinafter be called "annealing stain resistance" or "mill-cleaning property"); and
(ii) The rolling mill oil is supposed to show good lubricity upon rolling and not to develop galling called "heat streak" or vibrations called "chattering" (this property will hereinafter be called "lubricity").
In order to improve the mill-cleaning property (i), it is currently practiced to use such rolling mill oils that have been obtained by reducing the contents of fatty acids, fatty oils, fats and/or high-molecular organic compounds, which tend to lead abundant residual carbon components upon annealing, as much as possible and instead incorporating volatile or readily-decomposable materials such as mineral oils and synthetic esters as principal ingredients.
However, such rolling mill oils have poor adherence to materials, show weak oil-film forming properties in roll contact arcs and as a matter of fact, have inferior lubricity. An attempt for improvement to the lubricity (ii) will result in use of a fatty oil, fat or fatty acid at a high content such as beef tallow base rolling mill oil, leading to a reduction to the mill-cleaning property.
A mill-cleaning rolling mill oil, which is employed to omit the cleaning step, is hence required to have these mutually-contradictory two properties. Accordingly, mill-cleaning rolling mill oils which have been put into practical use to date are applied only to sheet gage materials which have relatively great finished plate thicknesses and permit use of mild rolling conditions (for example, those having finished plate thicknesses of 0.8 mm and greater).
With the foregoing in view, the present inventors carried out an extensive research to provide a cold-rolling lube oil equipped with both of the above-described properties. As a result, it was found that a composition obtained by mixing a specific amount of a monoester obtained from an aliphatic carboxylic acid and an aliphatic alcohol, a prescribed amount of a dimer acid or polymer acid and a predetermined amount of an ester obtained by reacting remaining carboxyl or hydroxyl groups of a polyester, which had in turn been obtained by heating and condensing at least one of dimer acids and/or polymer acids of unsaturated higher fatty acids with a polyol, with an alcohol or fatty acid did not develop oil stain and had excellent mill-cleaning property and good lubricity, on which a patent application has already been made (now, Japanese Patent Laid-Open No. 33395/1984).
Reflecting rapid advancement in rolling mills and rolling technology in recent years, the rolling speed has been increased to achieve mass production. Coupled with such advancement, severer requirements have been imposed on cold-rolling lube oils. Under the circumstances, conventional cold-rolling lube oils cannot fully meet such requirements. Conventional cold-rolling lube oils were however accompanied by one or more problems. Namely, such conventional cold-rolling lube oils are subject to thermal decomposition and thermal oxidative decomposition and are thus deteriorated during their applications in coolants under severe conditions to which they are believed to be exposed during actual cold rolling. Further, iron powder, scum and the like are formed during rolling work. These stain-forming impurities are then caused to mix in the lube oils. When rolled steel coils are subjected to subsequent steps with these stain-forming impurities still adhered together with the lube oils on the surfaces of the coils, the lube oils are polymerized and resinified and are thus rendered hard to evaporate due to chemical reactions such as oxidation, decomposition and polymerization under such conditions as mentioned above even if the lube oils are inherently supposed to evaporate substantially in their entirety by remaining heat of about 130.degree. C. or so and their heating to 200.degree.-300.degree. C. upon annealing. When the coils are then subjected to the subsequent annealing step, the lube oils are carbonized due to intensive heat (300.degree.-700.degree. C.), and stains of soot are formed on the entire surfaces of the steel sheets and soot is caused to stick on edge portions of the rolled steel sheets to develop such a state as the so-called "edge carbon" which causes poor appearance. Moreover, the above-mentioned resinified lube oils impair the easiness of surface treatments such as platability, bonderizability, paintability, etc.